CMake: clean up WASI SDK sysroot paths (#39275)
CMake: clean up WASI SDK sysroot paths (#39275)

Paths to WASI SDK sysroot should not assume `/share/wasi-sysroot` directory hierarchy. It's much more flexible to have this part hidden under the more general `SWIFT_WASI_SYSROOT_PATH` variable.
atrick/swift,roambotics/swift,atrick/swift,benlangmuir/swift,glessard/swift,ahoppen/swift,gregomni/swift,rudkx/swift,glessard/swift,xwu/swift,xwu/swift,ahoppen/swift,glessard/swift,JGiola/swift,ahoppen/swift,atrick/swift,glessard/swift,apple/swift,benlangmuir/swift,roambotics/swift,gregomni/swift,glessard/swift,benlangmuir/swift,apple/swift,xwu/swift,rudkx/swift,atrick/swift,xwu/swift,roambotics/swift,benlangmuir/swift,apple/swift,roambotics/swift,rudkx/swift,benlangmuir/swift,ahoppen/swift,gregomni/swift,gregomni/swift,JGiola/swift,xwu/swift,rudkx/swift,ahoppen/swift,benlangmuir/swift,apple/swift,JGiola/swift,ahoppen/swift,JGiola/swift,xwu/swift,rudkx/swift,atrick/swift,atrick/swift,rudkx/swift,JGiola/swift,apple/swift,gregomni/swift,xwu/swift,apple/swift,JGiola/swift,roambotics/swift,roambotics/swift,gregomni/swift,glessard/swift